The present invention generally relates to a method for controlling the surface character of bi-axially textured alloy substrates. The invention more particularly relates to the use of intermediate epitaxial films on textured metal substrates. More specifically, the present invention relates to the use of an epitaxial film deposited on a biaxially textured alloy substrate, the epitaxial film serving to stabilize the substrate surface against oxidation for subsequent deposition of an epitaxial film.
Some high temperature superconductors require well-aligned crystallites with low angle grain boundaries in order to yield high critical currents at relatively high temperatures. For example, thin films consisting of polycrystalline yttrium-barium-copper-oxide superconductors (YBCO) can yield a critical current density exceeding 106 A/cm2 at 77K, self field, when a substantial portion of the local grain boundary misorientations in the film are well below 10xc2x0, thereby mitigating the well known weak link behavior in current transport across boundaries between misoriented regions. This requirement for achieving high critical current densities in polycrystalline films can also be expressed as the YBCO film having a bi-axial texture in which, for example, the c-axis of each crystallite is substantially perpendicular to the film surface and the collective a-axes of all crystallites align in substantially the same direction in the plane of the film surface. To obtain an YBCO thin film with a good (for example,  less than  less than 10xc2x0 for a single crystal substrate) bi-axial texture, it can be deposited in an epitaxial manner on an oxide single crystal such as LaAlO3. This is not a commercially viable process for many applications since single crystal substrates cannot be economically produced in very long lengths or large areas. It is therefore more appropriate for industrial purposes to consider the epitaxial deposition of YBCO on a bi-axially textured buffer layer (often an oxide layer), which in turn has been deposited on a flexible metal substrate tape.
The flexible metal substrate can be used to provide a necessary template for texture and mechanical stability during handling and use in applications. Face centered cubic (fcc) metals and some alloys based on fcc metals are especially useful for substrate material, as they can be bi-axially textured using well known rolling deformation and annealing processes. A well-known texture in these metals and alloys is the so called xe2x80x9ccube texturexe2x80x9d, in which the c-axis of the substrate crystallites is substantially perpendicular to the substrate surface, and the a-axes align primarily along the tape direction. The cube texture can often be made with very low full-width at half-maximum (FWHM) values obtained from X-ray pole figures, an indication of collective alignment of both c- and a-axes of all crystallites. Under controlled rolling and annealing processes, these deformation textured metal tapes possess texture approaching that of single crystals. In practical application of the process, the FWHM texture is less than 10 degrees and more typically less than 8 degrees.
Nickel (Ni) is one fcc metal that can be made into thin substrates with a well-defined cube texture using the rolling and annealing process. Prior work has shown that oxide buffer layers can be deposited on a biaxially textured nickel surface using conditions under which nickel oxide is not stable, but the buffer layer (for example, CeO2 or Y2O3) is stable, allowing the oxide to inherit the texture of the underlying nickel substrate (that is epitaxy). A limitation to use of Ni, however, is its ferromagnetic character that may preclude its use in superconducting tape for alternating current (AC) applications such as power transmission cables, motors, and transformers. In addition, Ni is mechanically weak following the typical annealing heat treatment used to form the cube texture.
For these reasons, Ni alloys and other alloys have been developed to make strong, non-magnetic bi-axially textured substrates. These alloys often have alloying elements such as chromium (Cr) or aluminum (Al) that have a tendency to form stable oxides under very low oxygen partial pressure (PO2). The growth of epitaxial layers on metal or alloy substrates is commonly carried out under PO2 of less than 10xe2x88x9217 Torr and 650xc2x0 C., where the constituent elements such as Cr or Al will form surface oxides. The presence of these surface oxides can inhibit the growth of the epitaxial layers. This can be explained in part due to the fact that the surface oxides are typically randomly oriented on the textured alloy surface and can therefore interfere with a high quality epitaxial buffer layer deposition.
Texturing in HTS films has been demonstrated by epitaxial growth of the superconductor on appropriate templates. As used herein, xe2x80x9cepitaxialxe2x80x9d means that the crystallographic orientation of the superconducting film is derived from and directly related to the crystallographic orientation of the underlying template. Early work used single crystal oxide substrates as the HTS growth template. For many practical applications, the substrate must be flexible. A well-oriented template can be achieved by means of ion beam assisted deposition (IBAD) of oxide buffer layers on random polycrystalline metal substrates. High quality epitaxial superconducting films have been grown on such tapes. In alternative and preferred embodiments, texturing in substrates can be induced using the deformation texturing (xe2x80x9cDeTexxe2x80x9d) process (as set forth herein) and epitaxial deposition of buffer layer(s) and subsequent functional layer(s) can be accomplished as also described herein.
One major issue therefore relating to the epitaxial deposition of oxide buffer layers on a bi-axially textured substrate, whether a metal or an alloy, is the control of the oxygen partial pressure, or PO2. This is true for any buffer deposition technique, whether it is Physical Vapor Deposition (PVD) by pulsed lasers, sputtering, electron beam, or thermal evaporation, or by a non-vacuum process such as Metal-Organic-Deposition (MOD). The objective is to avoid formation of native oxide films on the surface of the substrate, thereby allowing the deposited buffer layer to nucleate and grow with the appropriate biaxial texture from the substrate surface. Some metals such as silver (Ag) have a natural ability to allow for growth using a great variety in PO2 conditions, but suffer from other disadvantages such as being difficult to texture, having a large coefficient of thermal expansion (CTE), high price, low mechanical strength and the like. Others such as copper (Cu) will easily oxidize. In some metals (for example, Ni), the PO2 can be carefully controlled at the deposition temperature to provide sufficient oxygen to stabilize the buffer layer but insufficient to oxidize the Ni due to basic thermodynamic considerations. Methods to control PO2 below the thermodynamic stability limit for NiO formation have been developed. For this reason, Ni has been established as a good deposition surface for epitaxial oxide layers such as oxides of the rare earth metals (yttria, ytterbia, ceria and the like). As discussed above, however, a pure Ni substrate suffers from other deficiencies that preclude its use in various applications.
In the context of surface control of substrates, it would therefore be desirable to provide controlled methods and articles for producing a surface on nonmagnetic, high strength biaxially textured substrates upon which an epitaxial layer can be deposited, thereby overcoming the shortcomings associated with the prior art.
A method has been established to mitigate stable oxide formation until a first epitaxial (for example, buffer) layer has been produced on the biaxially textured alloy surface.
It is therefore an object of this invention to provide an improved process for producing biaxially textured epitaxial (for example, buffer) layers on biaxially textured metal or alloy substrates.
It is another object of the present invention to provide an economical and commercially viable process for the deposition of epitaxial thin films on nonmagnetic and biaxially textured alloy substrates suitable for use in scale-up and manufacturing processes.
It is another object of the present invention to provide a process for depositing high performance high temperature superconducting films on various substrates and epitaxial layers suitable for use in scale-up and manufacturing conditions.
The present invention accomplishes these and other objectives by providing a method for forming epitaxial (for example, buffer) layer films on the surface of a biaxially textured, nonmagnetic substrate having an element or elements which are oxide-scale forming. The invention also includes methods for forming composite articles having multiple layers, one or more of which may be incorporated into an alloy substrate and having substantial alignment both in-plane and out-of-plane.
In one aspect, the invention includes a method of forming a composite article by forming a first film (for example, Ni) which is epitaxially deposited onto a biaxially textured alloy substrate and which prevents the formation of surface oxides on the biaxially textured alloy substrate during the initial deposition of a second film and wherein the first film provides a template for the growth of the second film. The first film is typically a metal such as Ni which is low cost and upon which epitaxial layers can be deposited in a controlled environment under which nickel oxide is not thermodynamically stable. In some embodiments, the Ni film would subsequently be incorporated into the substrate alloy by interdiffusion of Ni into the alloy and the elements of the alloy into the Ni where its ferromagnetic character would be eliminated or effectively eliminated to allow for use in various applications and the protective oxide scale could then form from the elements present in the substrate alloy beneath the epitaxial deposited buffer layer. The purpose of the interdiffusion is to substantially eliminate the negative aspects of Ni as a metal layer (for example, ferromagnetism, oxidation) during subsequent processes and use, and not for the purposes of forming a substrate alloy.
In another aspect, the invention features an article having a biaxially textured alloy substrate, a first metal film which is epitaxial on the substrate surface and which inhibits the formation of oxide surface scales during a controlled high temperature environmental exposure, such exposure being typical of initial buffer layer deposition.
In yet another aspect, the invention features an article having a biaxially textured alloy substrate, a buffer layer having a first biaxially textured thin film that was formed on a transient metal layer and which has since been incorporated into the alloy substrate, and a second film which has been epitaxially deposited on the first biaxially textured thin film. The second film, and possibly subsequent films can be another buffer layer material. In addition or alternatively, the second film may include materials selected from superconductors, semiconductors, photovoltaic materials, magnetic materials, capacitors and precursors of superconductors.
In one embodiment, the alloy substrate is a Cu-Ni-Al alloy that is biaxially textured by rolling deformation and annealing. The native aluminum-oxide scale which forms during the texture anneal can be removed by, for example, etching or sputtering. The substrate alloy alignment is preferably less than about 10 degrees FWHM in a {111} phi-scan. An epitaxial nickel layer is deposited on the surface of the substrate alloy to a thickness of about 0.2 microns. A first buffer layer is deposited upon the epitaxial nickel layer using for example electron beam evaporation. The buffer layer material is preferably CeO2 or Y2O3. The buffer layer is deposited typically at 650xc2x0 C. in a background environment of Ar/5% hydrogen at less than 200 mTorr. The buffer layer is typically less than about 20 nm in thickness. This first epitaxial buffer layer film will provide a template for the growth of subsequent epitaxial layers as necessary to produce a commercial product. During the growth of this and subsequent epitaxial layers at elevated temperatures (for example, 650-725xc2x0 C.), the Ni layer will be gradually incorporated into the substrate alloy leaving the epitaxial buffer layer adherent to a substantially homogenous substrate.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.